1. Field of the Invention
The invention relates to a premix burner for producing a homogeneously distributed fuel-air mixture for firing a combustion chamber which is used to drive a gas turbine which follows the combustion chamber.
2. Discussion of Background
The technique known as premix combustion has established itself in the combustion of liquid or gaseous fuel in a combustion chamber of a gas turbine. In this process, fuel and combustion air are premixed as uniformly as possible and are then passed into the combustion chamber and ignited. To comply with ecological aspects, a low flame temperature is maintained by means of a high excess of air. In this way, it is possible to keep the formation of nitrogen oxide at a low level.
A typical premix burner is known, for example, from EP-387 532 A1. Premix burners of this type are what are known as double cone burners which substantially comprise two hollow, conical part-bodies which are interleaved in the direction of flow. In this arrangement, the respective center axes of the two part-bodies are offset with respect to one another. In their longitudinal direction, the adjacent walls of the two part-bodies form tangential slots for the combustion air which passes into the interior of the burner in this way. A fuel nozzle for liquid fuel is arranged there. The fuel is injected into the hollow cone at an acute angle. The conical liquid fuel profile which is generated is surrounded by the combustion air flowing in tangentially. The concentration of the fuel is continuously reduced in the axial direction as a result of its mixing with the combustion air.
The premix burner can also be operated with gaseous fuel. For this purpose, gas inflow openings, known as the premix holes, which are distributed in the longitudinal direction, are provided in the walls of the two part-bodies in the region of the tangential slots. In gas mode, therefore, the mixture formation with the combustion air commences as early as in the zone of the inlet slots. It will be understood that in this way mixed operation with both types of fuel is also possible. At the burner outlet, a fuel concentration which is as homogeneous as possible is established over the annular cross section which is acted on. A defined spherical cap-shaped backflow zone is formed at the burner outlet, the ignition taking place at the tip of this zone, known as the flame front.
It is known from various documents, for example Combust. Sci. and Tech. 1992, Vol. 87, pages 329 to 362, that with a perfectly premixed flame the size of the backflow zone, which is equivalent to what is known as the flame stabilization region, has no influence on the nitrogen oxide emissions. On the other hand, however, the carbon oxide emissions and the emissions relating to unsaturated hydrocarbons (UHC) and in particular the extinction limits of the respective premix burners are greatly influenced by the size of the backflow zone. This means that the larger the backflow zone is designed to be, the greater the carbon oxide emissions, the emissions relating to unsaturated hydrocarbons and the extinction limit become. The result of this is also that with a larger backflow zone it is possible to cover a wider load range of the premix burner without the flame being extinguished. In addition to the size of the backflow zone, which, as explained above, has a decisive influence on the method of operation of the individual premix burners, the fuel distribution, i.e. the mixing profile of the fuel/air mixture, also has a major role in the area of flame stabilization.
If the above-described premix burner is supplied with premix gas uniformly along the premix holes, i.e. as part of a single-stage premix mode, stability problems result within the backflow zone which forms and the associated flame front if the fuel mass flow drops, for example when the gas turbine is operated in the lower load range. At the same time, the lower fuel mass flow also causes the depth of penetration of the premix gas supply to decrease along the premix injection, so that the core zone of the flame front which is formed in the shape of a spherical cap becomes leaner within the burner. The instability which then occurs can extinguish the flame. To achieve improved flame stabilization under these operating conditions, the premix burner is switched over to what is known as “pilot mode” in which gaseous fuel is injected along the premix burner in the vicinity of the central fuel nozzle. However, a pilot mode of this nature leads to the formation of a diffusion flame, with the result that very high exhaust-gas values, in particular very high NOx emissions, are reached. If the premix burner is operated in what is known as mixed mode, which is distinguished by fuel being injected both through the premix stage and through the pilot stage, combustion chamber pulsations increasingly occur in addition to the abovementioned increased exhaust-gas values, increasing the risk of a flashback into the premix burner region.